1. Field of the Invention
This invention relates to an anti-skid control system for motor vehicles, which is adapted to prevent the wheels of the motor vehicle from skidding during braking operation.
2. Description of the Prior Art
Generally, with an anti-skid control system for motor vehicles, anti-skid control is effected by means of microcomputers such that hold valves and decay valves comprising electromagnetic valves are opened and closed on the basis of electrical signals representing wheel speeds sensed by wheel speed sensors, thereby increasing, holding or reducing the brake pressure, for the purpose of securing improved steering performance and running stability of the motor vehicle, while at the same time shortening the braking distance.
FIG. 1 of the accompanying drawings illustrates, by way of example, the manners in which wheel speed Vw, wheel acceleration +Vw, wheel deceleration (negative acceleration) -Vw and brake pressure are varied during the operation of the conventional anti-skid control system, together with hold signal HS and decay signal DS for opening and closing hold valves and decay valves.
Under such a condition that the hold signal HS and decay signal DS are both interrupted so that the hold valves remain opened while the decay valves remain closed, if braking operation is started at time t1, then the brake pressure Pw is increased from the time t1 onward so that the wheel speed Vw is decreased and thus the deceleration -Vw thereof is gradually increased in absolute value. A reference wheel speed Vt is set up on the basis of the wheel speed Vw in such a manner that it is lower than the wheel speed Vw by a predetermined amount .DELTA.V and follows the wheel speed Vw with such a speed difference and it linearly decreases with a gentle gradient of -1G from time t2 when the wheel deceleration -Vw reaches a predetermined threshold level, say -1G.
At time t3 when the wheel deceleration -Vw reaches another predetermined threshold level -Gmax, the hold signal HS is interrupted, and thus the brake pressure Pw will be held at the value occurring at the time t3.
The wheel speed Vw is further decreased and becomes equal to the reference wheel speed Vt at time t4; thereupon the decay signal DS is generated, and reduction of the brake pressure Pw is started. As a result, the wheel speed Vw is changed from deceleration to acceleration at time t5, i.e., low-peak point. At time t6 when the wheel speed Vw recovered from the low peak by 15%, for example, of the difference between the wheel speed at the low-peak point t5 and the wheel speed Vw at the brake pressure reduction starting point t4, the decay signal DS is interrupted so that the decay valves are closed, and thus the brake pressure Pw is held at the value occurring at the time t6. The holding of the brake pressure Pw is continued until time t7 when a high peak of the wheel speed Vw is reached; at the high-peak point t7, the hold signal HS is interrupted so that the hold valves HV are opened and thus the brake pressure Pw is increased. In this case, the buildup of the brake pressure Pw effected from the time 7 onward is realized in such a manner that the brake pressure Pw is alternately increased and held in succession because of the hold signal HS being turned on and off intermittently so that the hold valves are opended intermittently; thus the brake pressure Pw is caused to build up gradually. At a time when the wheel speed Vw being decelerated becomes equal to the reference wheel speed Vt, like at the time 4, the decay signal DS is generated again, and thus reduction of the brake pressure Pw occurs. Similar control modes are repeated, and as a result, the vehicle speed is decreased gradually.
With the foregoing anti-skid control, when the vehicle runs on a road surface with a low friction coefficient .mu. (low-.mu. road surface), there is the tendency that the wheel speed is behind in recovery after the brake pressure is reduced; thus, a longer time is taken before the wheel speed Vw reaches a high peak which is the next pressure buildup starting point, or in some cases no high peak of the wheel speed occurs at all, as a result of which a non-braking condition persists all this while. To avoid persistence of such a non-braking condition, it has been the practice that a timer is provided which is adapted, when no brake pressure reduction stopping point is reached after a lapse of a predetermined time period, for forcibly interrupting the decay signal DS to close the decay vlaves and also interrupting the hold signal HS to open the hold valves, thereby causing the brake pressure Pw to be increased abruptly.
During the above-mentioned control operation, when the vehicle is running on a very low friction road surface such as frozen road surface, for example, it may happen that the wheel speed Vw of the wheel coupled to the driving axle is momentarily increased significantly over the vehicle speed for one reason or another, as indicated at K in FIG. 1. Such a momentary siginificant increase in the wheel speed will be referred to as "whisker" hereinafter. In such a case, the reference wheel speed Vt is also increased following the wheel speed Vw with the speed difference .DELTA.V, but when an abrupt decrease in the wheel speed Vw occurs subsequent to the above-mentioned momentary significant increase, the reference wheel speed Vt does not follow the wheel speed Vw and falls gently with the gradient of -1G; thus the wheel speed Vw and reference wheel speed Vt cross each other at time t8, as indicated at A in FIG. 1.
With the conventionl anti-skid control system, as mentioned above, when the wheel speed Vw becomes equal to the reference wheel speed Vt, this is judged, in the CPU of the microcomputer, as if the brake pressure reduction starting condition were met, so that the decay signal DS is generated, and thus reduction of the brake pressure Pw is started at time 8. However, this brake pressure reduction, unlike that started at the time 4, does not result in a low peak of the wheel speed Vw, and persists until time t9 when the decay signal is forcibly interrupted so that the decay valves are closed, by means of the timer which was started at the time 8. Specifically, assuming that the time set on the timer is 1.44 sec., the non-braking condition will persist for about 2 seconds, including the time period, say 0.6 seconds, from the time t9 to time 10 when the hold signal HS is turned off so that the hold valves are opened, by means of another timer started at the time t9. Obviously, this will lead to an increase in the braking distance.